How to set the JSTL variable value in java script?

<script>

 function function1()

 { 

  var val1 = document.getElementById('userName').value;

  <c:set var="user" value=""/>  // how do i set val1 here?   

 }
 </script>

How do I set the 'user' variable (JSTL) value from 'val1' (Java script)?

javascript jsp jstl
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